There are many different types of primary power sources available that convert fossil and other fuels into usable energy or power designed to perform work for one or more purposes. Some of the applications utilizing such power sources include everyday common items, such as motor vehicles, lawn mowers, generators, hydraulic systems, etc. Perhaps the best known example of a primary power source is the well known internal combustion engine, which converts the energy obtained or generated from the combustion of fossil fuel into usable energy, such as mechanical energy, electrical energy, hydraulic energy, etc. Indeed, an internal combustion engine has many uses both as a motor and as a power source used to drive or actuate various items, such as a pump. Converting fossil fuels into usable energy is also accomplished in large electricity plants, which supply electric power to power grids accessed by thousands of individual users.
While primary power sources have been successfully used to perform the several functions described above, they have not been successfully used independently in many applications because of their relatively slow response characteristics. Although large amounts of energy are contained within a single drop of fuel, internal combustion engines are particularly problematic in powering small devices, and particularly robotic devices and other similar systems that utilize a feedback loop to make real time adjustments in the movement of the mechanical structure being driven. In a robotic or any other system requiring rapid response, the power source typically must be able to generate output power that is capable of instantaneous or near instantaneous correction, as determined by the feedback received, that is necessary to maintain proper operation of the robotic device. Primary power sources utilizing fossil fuels for energy production have proved difficult or largely unworkable in these environments.
The response speed or response time of a power source functioning within a mechanical system, which response time is more accurately referred to as the system's bandwidth, is an indication of how quickly the energy produced by the power source can be converted, accessed, and utilized by an application. One example of a rapid response power system is a hydraulic power system. In a hydraulic system, energy from any number of sources can be used to pressurize hydraulic fluid, which pressurized fluid is stored in an accumulator for later use. This is what is meant by charging the accumulator. The energy contained in the stored pressurized fluid can be accessed almost instantaneously by opening a valve in the system and releasing the fluid in the accumulator for the purpose of performing work, such as extending or retracting a hydraulically driven actuator. The response time of this type of hydraulic system is very rapid, on the order of a few milliseconds or less.
An example of a relatively slow response power conversion system is the internal combustion engine, as discussed above. The accelerator on a vehicle equipped with an internal combustion engine controls the rotational speed of the engine, measured in rotations or revolutions per minute (“rpm”). When power is desired, the accelerator is activated and the engine increases its rotational speed accordingly. Setting aside impedance factors, the engine cannot reach the desired change in a very rapid fashion due to several inertial forces internal to the engine and the nature of the combustion process. If the maximum rotational output of an engine is 7000 rpm, then the time it takes for the engine to go from 0 to 7000 rpm is a measure of the response time of the engine, which can be a few seconds or more. Moreover, if it is attempted to operate the engine repeatedly in a rapid cycle from 0 to 7000 rpm and back to 0 rpm, the response time of the engine slows even further as the engine attempts to respond to the cyclic signal. In contrast, a hydraulic cylinder can be actuated in a matter of milliseconds or less, and can be operated in a rapid cycle without compromising its fast response time.
Once method of circumventing the slow response time of an internal combustion engine is to run the engine continuously at high speed, even when the pressurized fluid is not needed, and recycling the high-pressure fluid back into the intake reservoir through a throttle valve. In this configuration, a conventional piston pump continues to extract power with each pumping stroke as it forces the fluid into the high-pressure discharge line. If this high-pressure fluid is not needed by the system at that precise moment, it is directed to a bypass line which recycles the fluid back to the low pressure intake reservoir through a throttle valve, pressure orifice or other similar device that bleeds off the high pressure. Unfortunately, these pressure bleed-off devices cannot recover any of the work used to pressurize the fluid in the first place, so the energy is effectively lost. Operating in a high speed idle mode, therefore, is very disadvantageous for a conventional pumping system. It results in a great waste of power and energy whenever the pumped, high pressure fluid cannot be accommodated by the high-pressure side of the system.
To get around these inherent limitations with internal combustion engines, many applications require the energy produced by the primary power source to be stored in another, more rapidly responsive energy system capable of holding the energy in reserve so that the energy can be accessed later instantaneously. One example of such an application is heavy earth moving equipment, such as backhoes and front end loaders, which utilize the hydraulic pressure system discussed above. Heavy equipment is generally powered by an internal combustion engine, usually a diesel engine, which supplies ample power for the maneuvering and driving of the equipment, but is incapable of meeting the energy response requirements of the various functional components, such as the bucket or backhoe. By storing and amplifying the power from the internal combustion engine in the hydraulic system, the heavy equipment is capable of producing, in a rapid response, great force with very accurate control. However, this versatility comes at a cost. In order for a system to be energetically autonomous and be capable of rapid, precise control, more component parts or structures are required, thus increasing the size, weight and complexity of the system, as well as its attendant operating costs.
Another example of a rapid response power supply is an electrical supply grid or electric storage device such as a battery. The power available in the power supply grid or battery can be accessed as quickly as a switch can be opened or closed. A myriad of motors and other applications have been developed to utilize such electric power sources. Stationary applications that can be connected to the power grid can utilize direct electrical input from the generating source. However, in order to use electric power in a system without tethering the system to the power grid, the system must be configured to use energy storage devices such as batteries, which can be very large and heavy. As modern technology moves into miniaturization of devices, the extra weight and volume of the power source and its attendant conversion hardware are becoming major hurdles against meaningful progress.
The complications inherent in using a primary power source to power a rapid response source become increasing problematic in applications such as robotics. In order for a robot to accurately mimic human movements, the robot must be capable of making precise, controlled, and timely movements. This level of control requires a rapid response system such as the hydraulic or electric systems discussed above. Because these rapid response systems require power from some primary power source, the robot must either be part of a larger system that supplies power to the rapid response system or the robot must be directly equipped with one or more heavy primary power sources or electric storage devices. Ideally, however, robots and other applications should have minimal weight, and should be energetically autonomous, not tethered to a power source with hydraulic or electric supply lines. To date, however, technology has struggled to realize this combination of rapid response, minimal weight, effective control, and autonomy of operation.